Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. A typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) with catalyst layers on both sides. The catalyst coated PEM is positioned between a pair of gas diffusion media layers, and a cathode plate and an anode plate are placed outside the gas diffusion media layers. The components are compressed to form the fuel cell.
The currently widely used fuel cell electrocatalysts are platinum nanoparticles supported on carbon supports. Depending on the catalysts and loading, the electrodes prepared with carbon supported platinum catalysts normally have thickness from several microns to about 10 or 20 microns with porosities varying from 30% to 80%. One of the disadvantages of these carbon supported catalysts is the poor corrosion resistance of carbon under certain fuel cell operating conditions, which results in fast performance degradation.
The catalyst layers can be made of nanostructured thin support materials. The nanostructured thin support materials have particles or thin films of catalyst on them. The nanostructure thin catalytic layers can be made using well known methods. One example of a method for making nanostructured thin catalytic layers is described in U.S. Pat. Nos. 4,812,352. 4,940,854, 5,039,561, 5,175,030, 5,238,729, 5,336,558, 5,338,430, 5,674,592, 5,879,827, 5,879,828, 6,482,763, 6,770,337, and 7,419,741, and U.S. Publication Nos. 2007/0059452, 2007/0059573, 2007/0082256, 2007/0082814, 2008/0020261, 2008/0020923, 2008/0143061, and 2008/0145712, which are incorporated herein by reference. The basic process involves depositing a material on a substrate, such as polyimide, and annealing the deposited material to form a layer of nanostructured support elements, known as whiskers. One example of a material which can be used to form the nanostructured support elements is “perylene red” (N,N′-di(3,5-xylyl)perylene-3,4,9,10 bis(dicarboximide) (commercially available under the trade designation “C. I. PIGMENT RED 149” from American Hoechst Corp. of Somerset, N.J.)). A catalyst material is then deposited on the surface of nanostructured support elements to form a nanostructured thin film (NSTF) catalyst layer, which is available from 3M.
The nanostructured thin catalytic layers can be transferred directly to a proton exchange membrane, such as a Nafion® membrane, using a hot press lamination process, for example. The polyimide substrate is then peeled off, leaving the layer of whiskers attached to the membrane.
These types of nanostructured thin catalytic layers have demonstrated high catalytic activities, which is helpful to reduce the platinum utilization in fuel cell stacks. Most importantly, because the supporting layer is not made of carbon as in the traditional platinum catalysts for fuel cell application, the nanostructured thin catalytic layers are more resistant to corrosion under certain fuel cell operating conditions, and thus improve the fuel cell's durability.
However, after the annealing process is completed, a thin layer of residual non-crystallized perylene red remains at the surface of the polyimide substrate. In addition, the deposition of catalyst material can form a thin film of catalyst material between the whiskers. Therefore, when the whiskers have been transferred to the PEM and the polyimide substrate peeled off, the surface of the whiskers that was adjacent to the polyimide substrate is exposed and becomes the surface of membrane electrode assembly (MEA). Consequently, the residual non-crystallized perylene red backing, which originally was adjacent to the polyimide substrate, is exposed. This can be detrimental to the fuel cell operation because it can block water and gas transfer in and out of the electrode.
In addition, an MEA made with this type of whisker catalyst layer has a narrow range of operating conditions (i.e., they cannot be too dry or too wet) to provide good performance. If the fuel cell is operated under wet conditions, the thin layer of whiskers, which is less than 1 μm thick, cannot provide enough storage capacity for the product water, resulting in flooding. Under dry conditions, it is believed that not all portions of the whiskers are utilized to catalyze the reaction due to poor proton transfer characteristics.
Besides the NSTF whisker catalyst described above, there are other uniformly dispersed (or dispersed with a desired pattern) catalytic nanostructured materials prepared on a substrate. For example, aligned carbon nanotubes, aligned carbon nanofibers, or nanoparticles, and the like could be grown on silicon or other substrates. Catalytic materials are then deposited onto the nanostructured materials. Electrocatalyst decals incorporating such materials are described, for example, in Hatanaka et al., PEFC Electrodes Based on Vertically Oriented Carbon Nanotubes, 210th ECS Meeting, Abstract #549 (2006); Sun et al., Ultrafine Platinum Nanoparticles Uniformly Dispersed on Arrayed CNx Nanotubes with High Electrochemical Activity, Chem. Mater. 2005, 17, 3749-3753; Warren et al., Ordered Mesoporous Materials from Metal Nanoparticle-Block Copolymer Self-Assembly, Science Vol. 320, 1748-1752 (27 Jun. 2008).
In U.S. application Ser. No. 12/465,913, filed May 14, 2009, we described a different way to prepare restructured electrodes containing the nanostructured thin catalyst layer in the format of catalyst coated membrane (CCM). Depending the architecture of the fuel cell design, catalyst coated diffusion media (CCDM) sometimes has advantages over CCM. Gas diffusion media in PEM fuel cells is normally composed of a layer of carbon fiber paper or carbon cloth and a microporous layer (MPL) thereon. The microporous layer normally contains carbon powders and hydrophobic fluoropolymers. The microporous layer does not have strong inherent adhesive strength within itself and to the carbon fiber substrate. So it is difficult to transfer the nanostructured thin catalyst layer from its original carrying substrate to a diffusion layer through a decal transfer process as described in the CCM case.
Traditionally, CCDM is prepared by coating a catalyst-containing ink directly onto the microporous side of the gas diffusion layer. This method cannot be applied to nanostructured thin catalyst layers described above since the nanostructured thin catalyst layer is normally prepared on a carrying substrate.
Therefore, there is a need for processing and constructing an electrode containing various types of nanostructured thin catalytic layers which can provide good performance over a wider range of operating conditions.